The present invention relates to an imaging lens for forming an image of an object on an imaging element such as a CCD sensor and a CMOS sensor. In particular, the present invention relates to an imaging lens suitable for mounting in a relatively small camera, such as a camera to be mounted in a portable device, e.g., a cellular phone and a portable information terminal, a digital still camera, a security camera, a vehicle onboard camera, and a network camera.
In these years, in place of cellular phones that are intended mainly for making phone calls, so-called “smartphones” have been more widely used, i.e., cellular phones with such functions as those of portable information terminals (PDA) and/or personal computers. Since the smartphones in general are highly functional as opposed to the cellular phones, it is possible to use images taken by a camera thereof in various applications. For example, in case of the smartphone, while it is possible to print and enjoy images taken as in an intended use, it is also possible to use images in additional uses such as processing images to be used in games or for makeup simulations, dress fitting simulations, and the others. Such uses of the images, although not common previously, are becoming increasingly popular every year.
Generally speaking, a product group of cellular phones and smartphones is often composed of products with various specifications such as those for beginners and those for advanced users. Among them, an imaging lens to be mounted in the cellular phone or the smartphone, which is developed for advanced users, is required to have a high resolution lens configuration so as to be also applicable to a high pixel count imaging element of these days. However, when the imaging lens to be mounted in smartphones is used in the above-described usages, it is critical to have a small size and a wide angle of view, that is, a wide angle, rather than having a high resolution. Especially recently, as the smartphones have smaller sizes and higher functionalities, imaging lenses having a smaller size and a wider angle are more demanded.
Accordingly, in case of the cellular phones and smartphones, depending on their positions in the product group, specification of the imaging lens to be mounted in the cellular phones and smartphones is slightly different.
Although it is originally preferred to select a lens configuration most suitable for each required specification, it is also necessary to shorten a development period, reduce cost, etc., so that an imaging lens having a high resolution, a small size and a wide angle has been developed.
Since a lens configuration that is composed of five lenses has more flexibility in designing, such a lens configuration is expected to be applied to a next-generation imaging lens. Patent Reference, for example, has disclosed the imaging lens having the five-lens configuration. The imaging lens includes a first lens having positive refractive power, a second lens having negative refractive power, a third lens having positive or negative refractive power, a fourth lens having positive refractive power, and a fifth lens having negative refractive power, arranged in the order from an object side.
In the configuration described in Patent Reference, the first lens is formed in a shape so as to direct a convex surface thereof to the object side, and the second lens is formed in a shape so as to direct a concave surface thereof to the image plane side. In addition, the fourth lens is formed in a shape so as to direct a convex surface thereof to the image plane side and the fifth lens is formed as an aspheric shape having an inflexion point, which directs a concave surface thereof to the image plane side. Among them, the first lens and the second lens are made of lens materials having different Abbe's numbers such that a difference thereof becomes between 20 and 70. Accordingly, chromatic aberrations thereof are satisfactorily corrected, also with the refractive power arrangement and shapes of the respective lenses. According to the imaging lens described in Patent Reference, an aspheric surface is suitably applied, so that an incident angle of a light beam emitted from the imaging lens to an imaging element is also restrained and reduction of the peripheral light quantity due to shading is also restrained.
Patent Reference: Japanese Patent Application Publication No. 2012-113311
According to the imaging lens described in Patent Reference, a total length of the imaging lens tends to be elongated. Accordingly, although it is possible to obtain relatively satisfactory aberration, it is difficult to sufficiently satisfy demands for downsizing in these years. In addition, cameras equipped with the above-described imaging lens have been used in more various ways each year. Therefore, it is more demanded that the imaging lens has a small size and is applicable to a high resolution, which is accompanied with a higher pixel count of an imaging element, as well as having a wide angle so as to be applied for a wide imaging area. With the lens configuration described in Patent Reference, it is difficult to satisfy those demands in a well-balanced manner.
In view of the above-described problems in the conventional techniques, an object of the present invention is to provide an imaging lens that can attain a wide angle despite of a small size and effectively correct aberrations.
Further objects and advantages of the invention will be apparent from the following description of the invention.